Our long term goal is to learn how neurons find, recognize, and establish connections with their appropriate target cells. We are addressing these questions through a study of the development of the initial connections between the visual organs and the brain in Drosophila. These connections are made at the mid-point of embryonic development. We have identified a gene, disconnected, which is required for the establishment of these connections. We have begun to analyze the nerves in wild-type animals to determine whether specific "guideposts", or other landmarks can be recognized along the path taken by the larval optic nerves that may be necessary; for guidance and establishment of proper connections. This analysis involves determining the terminal connections established by the nerves and identifying potential target cells in the developing brain. We perform parallel analyses in disconnected mutant animals to determine whether specific defects in the contacts made by larval optic nerves in mutant animals can be associated with the mutant phenotype. The larval optic nerves are studied by confocal and high voltage electron microscopy. The nerves are identified by expression of antigens specific to the optic nerves and recognized by available monoclonal antibodies, by expression of lacZ in the nerves, and by their characteristic morphology. We have begun our work using Drosophila embryos which have been well preserved using ultra rapid freezing and freeze substitution and have obtained images of serial semi-thick sections. We have also made efforts to localize antigens of larval optic nerves in thin sections so that we might verify our observations of thick sections.